There are known, as output devices of computers and workstations, an image forming apparatus employing an ink-jet system in which ink is ejected to form an image on a recording medium, and an image forming apparatus employing an electrophotography. Of those apparatuses, as one having scanning means for scanning in a predetermining direction, in which the scanning means is scanned in a predetermining direction to form an image on a recording medium, for example, there is widely known an image forming apparatus employing an ink-jet system.
The image forming apparatus employing an ink-jet system comprises, for example, a print head for ejecting ink, a carriage on which the print head is mounted, said carriage reciprocating (scanning) in a predetermined direction, and a recording medium conveying device for conveying recording medium in a direction intersecting perpendicularly to the predetermined direction.
In the event that an image is recorded on a recording medium, the recording medium in the course of conveyance by the recording medium conveying device is temporarily stopped, and while the carriage is reciprocated in the predetermined direction, ink is ejected from the print head in accordance with an image signal carrying image information to form a band of image on a portion located at an image formation area of the recording medium on which area images are to be formed. Thereafter, the recording medium is fed by a band of width and stopped, and then again while the carriage is reciprocated in the predetermined direction, ink is ejected from the print head in accordance with the image signal carrying image information to form an image on a new portion located at the image formation area, of the recording medium. Such a performance is repeated to form an image.
Width (corresponding to a size with respect to a scanning direction of the carriage) of the scanning type image formation apparatus as mentioned above is generally determined by width of the recording medium and width of the scanning means. The shorter width of the scanning type image formation apparatus is more convenient in conveyance and installation. In order to shorten this width, the width of the scanning means may be shortened. However, there is a limit in shortening of width.
Now, for example, in case of a color image formation apparatus employing an ink-jet system, a carriage serves as scanning means. The carriage is loaded with at least four print heads. Thus, the carriage has an extended width per se. Further, it is needed for the color image formation apparatus to provide a registration with great accuracy, that is, an exact overlapping of the respective colors. For this reason, it happens that the carriage is loaded with a detection device for detecting a position of image formed with ink of each of the print heads at the time of the initial set up or an exchange of the print heads. In this case, it is usual that the detection device is mounted on the carriage in such a manner that it projects on an edge with respect to the scanning direction of the carriage. Accordingly, width of the carriage is extended by the corresponding projection of the detecting device.